Presently, the standard technique for the surgical treatment of myopia is a keratorefractive procedure wherein either the shape of the cornea is altered or the refractive index of the cornea is surgically altered. There are many problems associated with this procedure. While many physicians are trained in keratorefaction, relatively few ever perform them and then not often. It thus becomes an intimidating procedure when performed. There are technical difficulties resulting from the microkeratome, the computer and the cryolathe, and many problems in learning to use the cryolathe. Additionally, there are possible complications such as infection, loss of endothelial cells, penetration and perforation. Moreover, in the best of hands, the predictability rate is only about 55 to 65 percent to plus or minus 2 diopters.
Generally, keratorefractive procedures encompass any surgical procedure performed on the cornea, be it with a knife or with laser, which in effect attempts to induce a refractive change. It thereby encompasses radial keratotomy, keratomileusis, keratophakia, epikeratophakia, and polysulfone corneal inlays. It also encompasses the Ruiz procedure for astigmatism or any other type of procedure.
All of the refractive surgical procedures share common optical problems: glare and photophobia, over-correction, under-correction, regular astigmatism, irregular astigmatism, loss of best corrected acuity, fluctuation in visual acuity, loss of progression of effect, and monocular diplopia. These are well known, as reported by Perry Binder in "Optical Problems Following Refractive Surgery", OPTHALMOLOGY (June 1986, v. 93, no. 6).
In sum, even for an experienced surgeon performing a high volume of such procedures, the risks and associated problems are great.
Accordingly, there is a need for a successful phakic myopic implant, where a corrective lens is implanted in the eye without removal of the natural-crystalline lens, thus avoiding the risks associated with keratorefractive procedures.
Implantation of an intraocular lens is a general procedure practiced in one form or another by probably 90 percent of the present ophthalmologists. It is a procedure in which they are experienced. Thus, if a successful lens and method for the phakic treatment of myopia could be developed, the incidence of eyes that would be lost or damaged in treating myopia would clearly be reduced.
Additionally, if a successful lens and method for the phakic treatment of myopia could be developed, the patients would retain their power of accommodation. A 20 year old has an average range of accommodation of about 10 diopters; a 50 year old might have a range of accommodation of about 2.0 diopters. With most accommodation occurring in the anterior lens capsule and with the synkinetic reflex of internal rectus convergence and myosis, a properly vaulted lens would allow for adequate accommodation.
An especially desirable result, were a successful lens and method for the phakic treatment of myopia developed, would be the predictability of success, which would probably be 95 to 98 percent of the time. A common optical complication of all keratorefractive procedures is a 35% to 45% over-correction or under-correction. Moreover, the response time would be only two to three weeks, much less than the response times after keratorefractive procedures. There would be no donor material required and no irreversible incisions in the visual axis.
As a result, it would be highly desirable to develop a lens and method for the phakic treatment of myopia which would work.
The process of implanting an intraocular lens in the anterior chamber of an eye without removing the natural-crystalline lens has been attempted on four previous occasions. The first attempt in the early 1950's was in Italy and employed a solid parabolic-shaped lens which was extremely thick peripherally and centrally. The technique was abandoned after only a few cases. There was no published description of any long term effect. The procedure was a failure owing to the type of lens and technique of implantation employed.
Additional attempts in the mid-1950's were made in Spain and West Germany. These employed a semi-flexible style intraocular lens. These particular lenses were placed into the eye and resulted in a great deal of movement of the lenses after placement. These semi-flexible lenses were made centrally of polymethylmethacrylate and had carrying loops made of supramide and/or nylon. It was not known at that time, but supramide and nylon have proven to be biodegradable substances. As biodegradation progressed, lenses moved and many eyes were lost. These lenses were likewise abandoned. The operative techniques were never revealed and the negative results were never published.
In a fourth attempt in the late 1950's at the phakic treatment of myopia, six cases of implantation in the eye of a solid Choyce Mark-style lens were reported in a 1964 textbook by Mr. Peter Choyce. Choyce stated that the results were fairly satisfactory for binocular myopia, but this procedure was never carried out to any degree after 1964.
The first and fourth attempts employed the same operative technique. Without the benefit of a microscope, a Graffe incision was made in the lateral cornea from approximately 7-11:00 o'clock. A suture was placed through the cornea and retracted nasally. The solid parabolic-shaped lens or the Choyce Mark VIII lens measuring 1.0 mm. greater than the "white-to-white" horizontal eye diameter was then placed into the eye in an "open sky" technique. A peripheral iridectomy was carried out and the eye was sutured.
The phakic treatment of myopia has not been attempted since 1964. It has never been attempted in the U.S.A. The prior existing lenses and techniques of implantation employed were unsuccessful for a wide variety of reasons, including the following:
(A) The lenses were extremely thick and very rigid. There was constant touch to the central and peripheral cornea and many cases of corneal dystrophy resulted although they were not reported in the literature;
(B) The intraocular lens employed could never pass the scrutiny of scanning electron microscopy. Today's lenses are capable of being highly polished, ultra-refined, and tested by scanning electron microscopy before they are sterilized and packaged. In addition, they can be extremely thin with very refined edging and optics. The chemical nature of the plastic is still the same--polymethylmethacrylate;
(C) The only semi-rigid lens employed was a solid polymethylmethacrylate lens with supramide and/or nylon loops. This lens simply did not fit the eye and most often was too short. Those that were of sufficient size ultimately became loose because of the biodegradation of the supramide or nylon as it became encrotched in the anatomic angle of the eye. The action of the blood vessels in the anatomic angle biodegraded the supramide and ultimately the lenses became loose and created havoc within the eye. It is common knowledge in the ophthalmic profession that over 200 eyes were lost in Spain using this lens and procedure.
Finally, modern techniques for the implantation of intraocular lenses in the eye utilize lens glides for assisting the insertion of the lens into the eye. The lens glide is inserted in an incision in the eye. The lens is slid along the lens glide into the eye and positioned. The lens glide is then withdrawn from the eye. Flat lens glides and envelope-type lens glides are used for this purpose. However, flat lens glides merely provide a flat working surface, but beyond this do not facilitate positioning of the lens. Envelope-type lens glides, on the other hand, are inefficient and cumbersome because the lens is slid into the eye through an envelope and accurate positioning of the lens is hampered by continual contact of the lens glide and positioning surgical instrument. An envelope-type lens glide is described by Osvaldo I. Lopez et al. in "An Intraocular Lens Carrier," AM INTRA-OCULAR IMPLANT SOCIETY JOURNAL (Fall 1983, v. 9, pp. 477-479).